Memory devices are commonly employed as internal storage areas in a computing device or other electronic equipment. One specific type of memory used to store data in a computing device is random access memory (RAM). RAM is typically used as main memory in a computing environment, and is generally volatile in that once power is turned off, all data stored in the RAM is lost.
A static RAM (SRAM) is one example of RAM. SRAM has the advantage of holding data without a need for refreshing. A typical SRAM device includes an array of individual SRAM cells. Each SRAM cell is capable of storing a binary voltage value that represents a logical data bit (e.g., “0” or “1”). One configuration for an SRAM cell includes a pair of cross-coupled devices such as inverters. The inverters act as a latch that stores the data bit therein, so long as power is supplied to the memory array. In six-transistor (6T) cell, a pair of access transistors or pass gates (when activated by a word line) selectively couples the inverters to a pair of complementary bitlines (i.e., a bitline true and bitline complementary). Other SRAM cell designs may include a different number of transistors (e.g., 4T, 8T, etc.).
The design of SRAM cells has involved a compromise between the read and write functions of the memory array to maintain cell stability, read performance and write performance. In particular, the transistors which make up the cross-coupled latch must be weak enough to be over-driven during a write operation, while also strong enough to maintain their data value when driving a bitline during a read operation. The access transistors that connect the cross-coupled inverters to the true and complement bitlines affect both the stability and performance of the cell.
In one-port SRAM cells, a single pair of access transistors are used for both read and write access to the cell. The gates are driven to a digital value in order to switch the transistors between an “on” and “off” state. The optimization of an access for a write operation would drive the reduction of the on-resistance (Ron) for the device. On the other hand, the optimization of an access transistor for a read operation drives an increase in Ron in order to isolate the cell from the bitline capacitance and prevents a cell disturbance.
This compromise between the read function and the write function for an SRAM becomes more of an issue as integrated circuits are scaled down in size. In particular, read and write margins of the SRAM cells, which measure how reliably the bits of the SRAM cells can be read from and written into, are reduced as the operation voltages of the integrated circuits are reduced with the down-scaling of the circuits. Reduced read and write margins may consequently cause errors in the respective read and write operations for the SRAM cells. Further, the transistors which make up the cross-coupled latch must be weak enough to be over-driven during a write operation, while also strong enough to maintain their data value while driving a bitline during a read operation.